1. Field of the Invention
The present invention relates to endless track vehicles, and more particularly to a track suspension and drive system for a snowmobile.
2. Description of the Prior Art
Snowmobiles typically have a front steering ski and an endless drive track carried by a subframe rearwardly of the ski, with the subframe being resiliently suspended from the main frame of the snowmobile. The track suspension system of a snowmobile is important because it determines, at least to some extent, the vehicle's shock-absorbing capability, stability, handling, traction, and power requirements.
Snowmobile track suspension systems are of three general types. One type is a bogie wheel suspension, in which sets of bogie wheels carried by axles mounted on a resiliently suspended subframe engage the lower run of the drive track so as to tend to maintain the track in driving engagement with the snow. The advantage of this system is that there is mostly only rolling friction, with very little sliding friction between the bogie wheels and the track, thereby requiring little, if any, lubrication. Typical bogie wheel suspensions are shown, for example, in U.S. Pat. Nos. 3,773,126 and 3,658,145.
A second type of track suspension system is the slide rail suspension, in which the lower run of the drive track travels in sliding engagement with a pair of longitudinally extending rails. Snow acts as a lubricant between the track and rails to reduce friction. The advantage of a slide rail suspension over a bogie wheel suspension is that the former tends to hold the track flatter against the snow in rough or variable terrain, which can be important when speed or power is required, such as in racing. Typical slide rail suspensions are shown, for example, in U.S. Pat. Nos. 3,690,394; 3,738,714; 3,974,890; and 3,758,170.
A third type of track suspension system is a combination of the slide rail and bogie wheel suspensions in a single subframe assembly. Bogie wheels act to hold at least portions of the lower run of the track away from the slide rail to reduce sliding friction. Yamaha and Scorpion are known manufacturers of combination slide rail-bogie wheel track suspension systems.
A good discussion of the various snowmobile track suspension systems appears in an article entitled "What's New in Suspensions", by Lars Nelson, on Pages 34-36 of the Jan. 1974 edition of Snowtrack Magazine.
In most known track suspension systems of either the bogie wheel, slide rail, or combination type, the bogie wheels and slide rails are carried by a subframe which is independent of the snowmobile's main frame but resiliently suspended therefrom, so as to provide shock-absorbing capability and thereby isolate the main frame from much terrain-induced shock. The track on such subframes is driven by a pair of drive sprockets mounted on a drive axle. The drive axle is carried by the main frame of the snowmobile, as shown in all of the aforementioned patents. While the track drive axle and sprockets are located at one end of the track, there is usually an idler axle carrying a set of idler or guide wheels at the opposite end of the track. Often, the idler axle is also carried by the main frame. Therefore, the resilient means which suspends the subframe from the main frame does not isolate the entire length of the track from the main frame, resulting in some shock being transmitted through the fore and aft portions of the track and its axles to the main frame.
With the drive axle and sometimes the idler axle being mounted on the main frame, upward deflection of the track-carrying subframe relative to the main frame as permitted by the resilient suspension changes the track configuration. This causes the track to develop slack, resulting in a so-called "ripple" effect and vibration which can cause a momentary loss of driving power and increase track wear.
In addition, with the drive axle and sometimes the idler axle being mounted to the main frame, the over-all length of the track must be substantial in order to lay a desired flat length of track on the snow. The overall length of the track affects the total weight of the vehicle, the amount of power required to drive the track, and the effective distance between the track and steering skis (wheelbase). In general, the greater the wheelbase, the greater will be the maneuverability and responsiveness of a snowmobile of any given length, at least at high speeds.
With existing snowmobile track suspensions having main frame-mounted drive axles, the engine must be mounted generally vertically above the drive axle because of space limitations on the main frame and limitations on over-all vehicle length, the latter being dictated by a desire to control total vehicle weight. However, when the engine is mounted above the drive axle the vehicle has a relatively high center of gravity, which is undesirable, particularly in racing, which is usually conducted on oval tracks.
In view of the foregoing, there is a need for an improved track suspension and drive system for snowmobiles which will eliminate or alleviate the foregoing disadvantages of existing such systems.